Method for producing electrodes

ABSTRACT

The invention relates to a method for producing an electrode, in particular a negative electrode, of an electrochemical cell having a metal substrate, wherein the method includes the steps of treating the metal substrate with UV irradiation and treating the metal substrate using an organic acid.

The entire content of priority application DE 10 2011 011 154.9 is herewith incorporated by reference into the present application.

The present invention relates to a method for the production of electrodes, particularly for negative electrodes, for electrochemical cells. These electrochemical cells may be preferably used for powering a vehicle having an electrical motor, preferably with hybrid drive and/or in “plug in” mode.

Electrochemical cells, in particular lithium secondary batteries are used for energy storage in mobile information equipment such as mobile phones, in power tools or electrically powered cars and in cars with hybrid drive, respectively, based on to their high energy density and high capacity. In these different applications, in particular in regard to powering automobiles, these electrochemical cells must meet high demands: high capacitance and energy density, which remains stable over a large number of charge and discharge cycles, while having as minimal a weight as possible.

In particular, the usefullife-time of electrochemical cells is often dependent on the aging of the electrodes, in particular the aging of the negative electrodes. In the aging process, electrochemical cells lose capacity and performance. This process takes place, to some extent, in most of the common electrochemical cells and is highly dependent on the operating conditions (temperature, storage conditions, state of charge, etc.) but also on the quality, and the processing of the materials during the manufacturing process of the electrochemical cell. Thus, high-quality processing of pure materials leads to long-lived electrochemical cells that age only little over a long period of time, and therefore loose little capacity and performance over time.

Since the purity of the materials used is often subject to physical or chemical limits, for example due to synthesis, a primary objective of a battery manufacturer is to obtain electrochemical cells of higher quality and therefore more durable electrochemical cells by means of optimizing the manufacturing process of the electrodes, such as disclosed in EP 2 006 942.

In particular, adhesion of the electrochemically active material to the surface of the metallic substrate is a key to the quality of a electrochemical cell. From the prior art. it is known that said adhesion of the electrochemically active material to the surface of the metallic substrate may be improved by means of a corona treatment, i.e. by means of etching the surface of the metallic substrate with chrome sulfuric acid. A major disadvantage of this method, however, is the use of chrome sulfuric acid, which is highly toxic to humans and the environment, and which also leads to contaminations in the further processing of the electrochemical cells, that cannot be tolerated.

In light of the prior art, one object of the invention is to provide an improved process (method) for the production of electrodes, particularly for negative electrodes of durable electrochemical cells.

This is achieved according to the teaching of the independent claims of the present invention. Preferred embodiments of the invention are the subject matter of the dependent claims.

To solve this problem, as described in detail in the following, a method for the manufacture of electrodes for electrochemical cells is provided, particularly for negative electrodes, which comprises the treatment of the metallic substrate by means of UV irradiation and by means of using an organic acid.

The advantage of the method is that an environmentally friendly and reliable cleaning of the metallic substrate is achieved, which, in particular, improves the adhesion of the electrochemically active material on the surface of the metallic substrate and leads to electrochemical cells, with a longer useful period of use (“lifetime”).

The term “electrochemical cell” is understood to mean any device for the electric storage of energy. The term therefore defines, in particular, electrochemical cells of the primary or secondary type, but also relates to other forms of energy storage devices, such as capacitors. A preferred electrochemical cell in accordance with the present invention is a lithium ion cell.

The term “negative electrode” means that the electrode provides electrons to a load, for example when connected to an electrical motor. Thus, the negative electrode is the anode in accordance with this convention. Correspondingly, the term “positive electrode” means that the electrode takes up electrons when connected to a load, for example to an electrical motor. Thus, the positive electrode is the cathode in accordance with this convention.

An electrode, i.e. a positive electrode and/or a negative electrode, which is produced by the method of the invention comprises at least a metallic substrate and at least an electrochemically active material.

The term “electrochemically active material” is understood to mean a material, which is suitable for the storage and supply of ions, especially cations, preferably of lithium ions.

In one embodiment, the electrochemically active material is a cathode active material.

In a preferred embodiment, the electrochemically active material is an anode material. The anode active material preferably contains carbon.

In one embodiment, the electrode as prepared in accordance with the invention comprises, in addition to the metallic substrate and the electrochemically active material (preferably, the anode active material) at least one further additive, preferably an additive for increasing the conductivity, such as a carbon-based material, such as carbon black, and/or a redox-active additive, which reduces, preferably minimizes, and preferably prevents the destruction of the electrochemically active material in the event of overload of the electrochemical cell.

The term “metallic substrate” preferably refers to the part of cell, which is known as “electrode support” or “collector”. In the present case, the metallic substrate is suitable for the application of active material, and is substantially metallic in nature, preferably completely metallic in nature.

Preferably, the metallic substrate is, at least partially, configured as a film or a net structure or a mesh/web (“Gewebe”), preferably comprising copper or a copper-containing alloy,

The inventive method preferably comprises a step, in which the metallic substrate, in particular the surface of the metallic substrate, is pretreated so that a time difference exists in regard to the application of the active material and the treatment with an organic acid, in particular, the substrate has been at least partially cleaned and preferably has been completely cleaned.

The term “time difference” means that between the treatment, in particular the at least partial cleaning of the metallic substrate, in particular of the surface of the metallic substrate, with an organic acid, and the application of the active composition onto the pretreated metallic substrate, a time difference dt>0 lapses. The treatment, in particular the at least partial cleaning the metallic substrate with an organic acid, takes place before the application of the active composition onto the pretreated metallic substrate. The time difference dt between the treatment, in particular the at least partial cleaning of the metallic substrate with an organic acid, and the application of the active composition onto the pretreated, in particular the at least partially purified collector, preferably is up to dt=3 hours, preferably up to 2 hours, preferably up to one hour.

In a preferred embodiment, the time difference between the pretreatment, in particular the at least partial cleaning of the metallic substrate, in particular of the surface of the metallic substrate with an organic acid, and the application of the active composition onto the pretreated, in particular onto the at least partially cleaned metallic substrate, is between 30 minutes and 40 minutes, preferably 35 minutes (+/−2 minutes).

This time difference between the treatment, in particular the at least partial cleaning of the metallic substrate, in particular of the surface of the metallic substrate, with an organic acid and the application of an electrochemically active material onto the pretreated, in particular an at least partially cleaned metallic substrate, is advantageous in that a particularly effective cleaning is possible, wherein, preferably up to 50% of the impurities are removed, and particularly preferably up to 100% of the metallic impurities are removed from the substrate, and in particular from the top surface.

In between the cleaning of the metallic substrate with an organic acid and the application of the electrochemically active material, further process steps for further treatment of the surface of the metallic substrate may be implemented, such as, for example, a drying step.

The term “organic acid” is to be understood to relate to a chemical compound, which has a chemical acid group O═X—OH, i.e. which has a central atom (X), to which an OH group is bound by a single bond between the central atom, X, and the O atom of the OH-group, and which comprises a further oxygen atom, which is bound to the central atom X by a double bond. The central atom of X may be selected from the group of non-metals or semi-metals of the periodic system of chemical elements (PSE), which are capable of binding to an oxygen atom through formation of a double bond, and simultaneously with the oxygen atom O of the OH group by formation of a single bond. Preferably, the central X atom is selected from the group of carbon, sulfur, phosphorus, silicon; carbon is particularly preferred.

Furthermore, the central atom of X is additionally bound to another atom, preferably a carbon atom which is part of an organic substituent, which is selected from alkyl or aryl, which substituent, in addition to carbon and hydrogen atoms, may comprise additional further heteroatoms, preferably nitrogen, oxygen, sulfur or phosphorus. The use of the term “organic acid” in the singular does not exclude that said organic acid may also be a mixture of various organic acids.

In case the organic acid is a “solid” acid, i.e. an acid, which is present, under standard conditions (25° C., 1.031 bar), as a solid, it is preferable to dissolve the acid before use in an appropriate solvent. Preferably, the organic acid and/or the solvent has a water content of less than 20%, preferably less than 10%, preferably less than 5%, preferably less than 2% and more preferably 1% or less. Preferably, the organic acid is chosen so that the same decomposes at these elevated temperatures or under UV irradiation. Preferably, the decomposition products resulting from the ultraviolet radiation treatment are at least partially gaseous. Preferred decomposition products are, for example, CO₂ or water.

In one embodiment, the organic acid is selected from acetic acid, succinic acid, fumaric acid, citric acid, maleic acid, oxalic acid, lactic acid, pyruvic acid, formic acid, oxal succinic acid, oxalic acid or mixtures thereof.

In a preferred embodiment, the organic acid—optionally together with other components—is oxalic acid (also called “ethane di-acid”).

The use of organic acid, in particular oxalic acid has the advantage that the organic acid can be degraded (decomposed) by, for example, heating or UV irradiation. The resulting decomposition products of the organic acids are mainly CO₂ and water, and can be disposed of or removed, respectively. In addition, the handling of organic acids in essentially simpler and less dangerous than dealing with, for example, chrome sulfuric acid, as used, for example, in the Corona etching process. This is particularly relevant in the case of “clean room” conditions, which prevail for the manufacture of electrochemical cells.

In a particularly preferred embodiment, the organic acid is provided as an “anhydrous” oxalic acid, which is commercially available under the CAS No. 144-62-7. “Water-free” means that the water content of oxalic acid is 1% or less.

In a further particularly preferred embodiment, the organic acid is provided as “anhydrous”, oxalic acid, and is at least partially dissolved in NMP (N-methyl-2-pyrrolidon), which preferably has a water content of less than 100 ppm (=parts per million), preferably less than 60 ppm, preferably less than 30 ppm, preferably less than 10 ppm and is provided free from impurities, and in so-called “Quality Battery”, that is, in essence, free of amine impurities.

The use of anhydrous organic acids, in particular of anhydrous oxalic acid has the advantage that impurities in the metallic substrate, in particular on the surface thereof, and in particular in case the metallic substrate is provided as a copper foil, may be removed efficiently and easily, at least partially, preferably completely. The contamination of the surface of the metallic substrate may be caused by storage, transport, packaging or may have been caused during the preparation of the metallic substrate. Contaminants may adversely affect, for example, the adhesion of electrochemically active material onto the surface of the metallic substrate, causing the electrochemical cell to “age” faster, or may even negatively affect the function of the metallic substrate, namely the uptake or release of electrons from or to the electrochemically active material, which negative effect may be manifested, for example, in the form of increased internal resistance and a consequent loss of performance or capacity of the electrochemical cell.

In one embodiment, the metallic substrate is or comprises copper or a copper-containing film, in particular copper foil, which is associated with the problem that the surface of the copper foil collector is often brought in contact with impurities during its manufacture, for example during a rolling process or cutting process, often with fatty and/or oily substances, in particular with beef tallow, or dust particles. Furthermore, the surface of the copper-containing film, in particular of a copper foil is at least partially passivated during prolonged contact with ambient air, i.e. by means of oxidation a passivation layer forms, which may comprise, in one embodiment, copper (I) oxide, Cu₂O, which is also considered as an impurity. Therefore, the use of organic acids having organic substituents proves to be advantageous because the organic fatty and/or oily substances (impurities) at least partially, preferably completely, dissolve in the organic acid, and thus can be removed from the surface of the metallic substrate in accordance with the chemical principle “similia similibus solvuntur” (similar dissolves similar). Another advantage of the use of organic acids is that the passivation layer, in one embodiment, comprising copper (I) oxide Cu₂O is at least partially, preferably completely, removable. Preferably, the so treated and cleaned surface, in particular the at least partially cleaned surface of the metallic substrate, does not undergo further reactions with the organic acid.

In a particularly preferred embodiment, the metallic substrate is realized as a copper-containing film, in particular as a copper foil, whose surface is at least partially contaminated with oily and/or fatty substances, in particular with beef tallow, and/or a passivation layer comprising at least partly, copper (I) oxide, Cu₂O, and is treated with an organic acid, preferably with anhydrous oxalic acid, at least partially, preferably completely, and is thereby, in particular, at least partially, preferably entirely, freed from these contaminants.

The term “cleaning” and “to clean” means that, preferably, up to 50%, preferably up to 70%, preferably up to 100% of the impurities are removed from the surface of the metallic substrate, however preferably at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, 85%, 90%, 95% of the impurities are removed from the surface of the metallic substrate.

The terms “treatment” and “treating” or “to treat” and “pretreat” are to be understood that preferably up to 50%, preferably up to 70%, preferably up to 100% of the surface of the metallic substrate have come into contact with organic acid and have been, in particular, wetted, wherein, in each case, preferably at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, 85%, 90%, 95% of the surface of the metallic substrate come into contact with an organic acid and are, in particular wetted with said organic acid.

The wetting of the surface of the metallic substrate with an organic acid takes place, in one embodiment, by means of spraying the surface of the metallic substrate with an organic acid.

In another embodiment, the wetting of the surface of the metallic substrate with an organic acid is achieved by sprinkling the surface of the metallic substrate with an organic acid.

In another embodiment, the wetting of the surface of the metallic substrate is achieved through an immersion bath of the metallic substrate in organic acid.

In another embodiment, the wetting of the surface of the metallic substrate with an organic acid is achieved by means of a device, for example of a roller, whose surface is wetted with an organic acid, wherein said organic acid is at least partially transmitted onto the surface of the metallic substrate by means of contact with the surface of said device.

In another embodiment, the treatment of the surface of the metallic substrate with an organic acid is achieved by exposing the metallic substrate to vapor deposition in a steam comprising or consisting of organic acid. This allows for a particularly uniform treatment of a surface, in particular essentially free of “wetting” or “de-wetting” effects. The treatment preferably takes place at temperatures of at least 85° C., 100° C., 150° C. The treatment may comprise a steam jet so that the steam applies a pressure on the surface to be treated, which causes a mechanical cleaning effect. The pressure is, in each case, preferably at least 1 bar, 2 bar, 5 bar, 10 bar, 25 bar, 50 bar, 100 bar, 200 bar or 500 bar, but the pressure or ambient pressure in regard to the metallic substrate may also be 1 bar. The cross section of the steam jet may have an area AD, which corresponds at least to AO, which is the area of the surface that is to be treated. However, it is also possible, and preferred, that this cross-section of AD corresponds to a fraction f of the area AO (AD=f*AO), and is, preferably at least, or at most, f=0.5, 0.25, 0.1, 0, 05. The cross section of the steam jet has, in each case, and preferably, a shape that is substantially a rectangle, a line or a strip.

The treatment of the metallic substrate, in particular the cleaning thereof, preferably makes use of a plasma, in particular a plasma stream, in particular at an ambient pressure of between 0.05 bar and 1 bar vis-à-vis the metallic substrate. Plasma is a gas that partially or completely consists of free charge carriers, such as ions or electrons, and is, for example, produced by electrical treatment of a gas in an electric alternating field, as obtained, for example in commercially available plasma systems. The plasma can be generated using oxygen or an organic acid. In this context, the temperature may be chosen freely, in particular substantially room temperature. The result is a more flexible and gentle cleaning.

It is then also possible, and preferred, that the organic acid, in particular the organic acid of a steam jet, and the surface to be treated are moved relative to each other, preferably at a constant speed, to achieve a more uniform result, preferably by means of, for example, by means of moving the surface to be treated against the organic acid (or the steam jet); alternatively, the organic acid (or the steam jet) is moved against the surface to be treated.

In one embodiment, the wetting of the surface of the metallic substrate is followed by a further process step, in which the organic acid is distributed evenly over the surface of the metallic substrate by means of mechanical operations, such as shaking.

In one embodiment, the uniform distribution of the organic acid over the surface of the metallic substrate takes place simultaneously with the wetting of the surface of the metallic substrate with an organic acid.

In one embodiment, the method comprises a mechanical cleaning of the surface of the metallic substrate, which can be achieved, for example, by means of applying friction by means of brushes or textile. The step of mechanical treatment may be implemented prior to the wetting the surface of the metallic substrate with organic acid, or also during the wetting of the surface of the metallic substrate with an organic acid, or also subsequently to the wetting.

In a preferred embodiment, the process steps of wetting the surface of the metallic substrate with an organic acid, of evenly distributing the organic acid on the surface of the metallic substrate, and the step of mechanical cleaning of the surface of the metallic substrate are combined in a single process step, which for example, may be implemented via a steam jet comprising the organic the acid, or may be implemented by the use of movable brushes, wherein the organic acid is taken from a storage container filled with the organic acid, and the surface can therefore be wetted continuously with the latter, and therefore apply the organic acid by means of contact with the surface of the metallic substrate. The movable brushes may perform, for example, circular motions on the surface of the metallic substrate, so that the organic acid is uniformly distributed over the surface of the metallic substrate. By means of optional application of pressure, via the brushes, on the surface of the metallic substrate, said substrate is cleaned, at the same time, mechanically.

The residence time of the organic acid on the surface of the metallic substrate is preferably up to 30 seconds, preferably up to 5 minutes, preferably up to 30 minutes, preferably up to 60 minutes, preferably up to two hours. The residence time may also be longer or shorter.

Furthermore, the inventive method comprises a treatment of the metallic substrate with ultraviolet light.

In one embodiment, the UV-irradiation of the metallic substrate takes place before the treatment with an organic acid.

In another embodiment, the UV-irradiation of the metallic substrate takes place after the treatment with an organic acid.

In a preferred embodiment, the UV-irradiation of the metallic substrate takes place before and after the treatment with an organic acid.

In a preferred embodiment, in addition to the metallic substrate, the electrochemically active material is also irradiated with UV light.

The irradiation of the electrochemically active material with UV light may be implemented immediately prior to the application of the active material onto the metallic substrate, or also with a predetermined time difference.

The UV treatment of the metallic substrate and of the electrochemically active material is advantageous in regard to the adhesion of electrochemically active material on the surface of the metallic substrate. By means of UV irradiation, organic contaminants may be, at least partially, removed from the surface, by means of oxidation. Decomposition products such as CO₂ and water may form in this case. Thus, the surface of the metallic substrate is at least partially purified, and the adhesive forces of the surface of the metallic substrate are at least partially increased by means of UV irradiation.

Another benefit of the UV treatment of the metallic substrate is that any organic acid that is potentially present on the surface of the metallic substrate—after the application step—may be removed by UV irradiation, at least partially, preferably completely, by decomposition of the organic acid. This effect is particularly advantageous in case the UV irradiation leads to volatile decomposition products, such as gases, such as CO₂. Furthermore, the formation of water as a decomposition product of the UV-irradiation is possible. These decomposition products may be easily removed in the further process and are not harmful in electrochemical cells.

Suitable sources of UV radiation are, for example, mercury vapor lamps, e.g. low-pressure mercury lamps, and UV light emitting diodes. UV light in the sense of the present invention is electromagnetic radiation having wavelengths of 1 nm to 380 nm.

Preferably, the method comprises the step that the metallic substrate is dried, so that any liquid adhered to the surface of the metallic substrate, in particular water, is reduced or removed. The drying step is performed before and/or after the treatment with UV light.

The drying step may furthermore be performed before and/or before and/or after the treatment with an organic acid.

Preferably, the inventive method comprises the step of coating the metallic surface with electrochemically active material, wherein said surface is preferably treated with UV irradiation, and/or with organic acid. It is also advantageous if the coating step is followed by a further treatment with UV irradiation.

In one embodiment, the electrochemical cell according to the present invention comprises at least one electrode, which was manufactured in accordance with the method of the present invention. The electrode is preferably a negative electrode, which comprises a metallic substrate, which is preferably configured to contain copper, and which is preferably realized as a foil/film, and the total surface area of which is preferably up to 30%, preferably up to 50%, preferably up to 70%, preferably up to 100% coated with electrochemically active material, preferably is integrally coated (in material contact), wherein with the electrochemically active material is carbonaceous, preferably selected from crystalline graphite or amorphous graphite or “hard carbon” or mixtures thereof.

In another embodiment, the electrochemical cell also comprises, in addition, a binder which is capable of improving the adhesion between the electrochemically active material and metallic substrate. Preferably, such a binder comprises a polymer, preferably a fluorinated polymer, preferably a poly-vinylidene fluoride, which is sold under the trade name Kynar®, Solef®, Kureha® or Dyneon®.

FIG. 1 shows a schematic embodiment of the method according to the invention.

The embodiment of the method for manufacturing an electrode in accordance with the present invention comprises the steps of:

Providing a metallic substrate (11), preferably a copper metal collector or a copper-containing collector. Subsequent treatment of the metallic substrate, in particular of the surface of the metallic substrate, by UV irradiation (12). Providing an organic acid, particularly an oxalic acid (21), which is then at least partially dissolved in NMP (22). Method steps (11) and/or (12) may be performed parallel to the steps (21) and/or (22). The steps (11) and/or (12) may also be performed independent of, or with a time difference vis-à-vis, steps (21) and/or (22). Thus, it is also possible, for example, that steps (11) and (21) have already been performed, then process step (22) is carried out and only thereafter step (12) is carried out.

Application of the organic acid, preferably dissolved in NMP, preferably anhydrous oxalic acid as dissolved in NMP, onto the UV-treated surface of the metallic substrate (30)

At least partial removal of the organic acid as applied in step (30), which is dissolved in NMP, off the surface of the metallic substrate (40). It is advantageous that step (30) takes place with a time difference vis-a-vis to method step (40). This allows for a more effective cleaning.

Treating the surface of the metallic substrate from process step (40), as cleaned with organic acid, with UV radiation.

Providing electrochemically active material, in particular electrochemically active material for the anode (71), and treatment of the electrochemically active material with UV radiation (72). Steps (71) and (72) may be carried out at any time. It is particularly advantageous, however, when the process of step (72) is carried with minimal time difference in regard to step (60), which step comprises the coating of the metallic substrate, in particular the surface thereof, as pretreated in accordance with previous steps (12), (30), (40) and (50).

Treating of the metallic substrate as coated with the electrochemically active material with UV radiation (80). Step (80) is an optional step. 

1-8. (canceled)
 9. A method for producing an electrode, in particular a negative electrode of an electrochemical cell, comprising a metallic substrate, wherein said electrode comprises at least one metallic substrate and at least one electrochemically active material, the method comprising: treating the metallic substrate by UV-irradiation; treating the metallic substrate with an organic acid, wherein the surface of the metallic substrate is treated with an organic acid, in particular is cleaned at least partly, with a time difference in regard to the application of the electrochemically active material.
 10. The method according to claim 9, wherein the metallic substrate comprises copper.
 11. The method according to claim 9, wherein the UV-irradiation of the metallic substrate is performed either before or after, or before and after, the treatment of the metallic substrate with an organic acid.
 12. The method according to claim 9, wherein the organic acid is comprised of water-free oxalic acid.
 13. The method according to claim 12, wherein the oxalic acid is dissolved in anhydrous NMP.
 14. The method according to claim 9, wherein the treatment of the metallic substrate with an organic acid is performed 30 to 40 minutes prior to coating of the metallic substrate with an electrochemically active material.
 15. The method according to claim 14, wherein the electrochemically active material is treated with ultraviolet irradiation.
 16. A method for cleaning a metallic substrate of an electrode, in particular a negative electrode of an electrochemical cell, the method comprising: treating the metallic substrate with UV radiation; and treating the metallic substrate with an organic acid. 